This invention relates to a composite memory device including a recording medium and a nonvolatile storage medium so that data may be written to and read out from it on the basis of a file system that is common to them and also to a data writing method and a data writing program for writing data to such a composite memory device.
Hard disc drives (to be referred to simply as HDDs hereinafter) are being popularly used as external storage devices for personal computers (PCs) and made to have larger memory capacities as a result of improvements on recording density. They have been finding and are being expected to find applications in the field of various consumer AV appliances including AV home servers and car AV appliances.
Additionally, the disc size of HDDs has been reduced and HDDs of a size as small as 1.8 inches or even 1 inch are expected to become widely available for mobile appliances such as digital still cameras (DSCs) and portable music players.
On the other hand, flash memories and other nonvolatile solid-state memories provide advantages including a low power consumption rate, a quick starting ability and a high impact-resistance and are being made to have larger capacities that are in excess of 1 GB. Thus, they can find various applications where the advantages can effectively be exploited.
Meanwhile, the requirements to be met by small storage devices that are applied to mobile appliances include low cost, a high storage capacity, a low power consumption rate and quick responsiveness.
However, HDDs require several seconds after the start of power supply to get to a ready state where data can be recorded to or reproduced from them. On the other hand, nonvolatile sold memory devices excel in quick responsiveness and become ready instantaneously after the start of power supply so that data can be recorded to or reproduced from them from the moment when power is supplied to the device.
Power is wasted when an HDD is held in an idle state (where it is ready for recording or reproducing data) to consequently reduce the efficiency of use of the power source in a mobile appliance whose power source has only a limited capacity. HDDs have an additional disadvantage that a replacing operation takes time to consequently give rise to a fall of the data transfer rate when a defective sector appears on a track.
Thus, so-called hybrid storage devices that are products of a harmonized combination of an HDD and a nonvolatile solid-state memory are expected to be developed so as to cover the disadvantages of HDDS with the advantages of nonvolatile solid-state memories. Inventors of the present invention have already proposed various hybrid storage devices that can be managed by a single file system to exploit their advantages (see, inter alia, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2003-123379, Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 2003-125358, Patent Document 3: Jpn. Pat. Appln. Laid-Open Publication No. 2002-150699 and Patent Document 4: Jpn. Pat. Appln. Laid-Open Publication No. 2000-324435).
In the past, the system data that are required when accessing the software and the data for controlling the HDD controller are stored in a predetermined area of the HDD and cannot be read out until the HDD comes into a ready state. In other words, the appliance containing the HDD cannot be used for a while after the start of power supply until the system data are read out from the HDD. In view of this problem, Patent Document 1 proposes a technique by means of which the system data are stored in the nonvolatile solid-state memory so that the appliance comes into a ready state the moment when power is supplied to the appliance.
As pointed out above, HDDs require several seconds after the start of power supply to get to a ready state where data can be recorded to or reproduced from them. In other words, data cannot be recorded to or reproduced from the HDD until it gets to a ready state. In view of this problem, Patent Document 2 proposes a technique by means of which data are written to the nonvolatile solid-state memory for a predetermined period of time from the start of operation and subsequently to the HDD since the time when the latter gets to a ready state in a data recording mode, whereas data are firstly read out from the nonvolatile solid-state memory and subsequently from the HDD since the time when the latter gets to a ready state in a data reading mode so that data can be recorded or reproduced from the moment when power is supplied to the appliance.
Patent Document 3 describes a technique for preventing a fall of the data transfer rate from taking place when a defective sector appears by using the nonvolatile solid-state memory as spare storage area.
When a video camera that is equipped with an HDD is inadvertently dropped or subjected to an impact while data are being recorded to the HDD, the power supply can be abruptly suspended to make the file system no longer operational for data registration. Then, it will be no longer possible to reproduce data from the HDD. Therefore, the file system needs to be periodically updated in a predetermined area of the HDD in order to prevent such a catastrophic situation from taking place. However the data recording rate of the HDD can be remarkably reduced by the arrangement for updating the file system. In view of this problem, Patent Document 4 describes a technique for assigning a storage area to be used for updating the file system to the nonvolatile solid-state memory so that data may be protected without reducing the transfer rate.
Thus, it is possible to provide a low cost high performance storage device by combining a small capacity nonvolatile solid-state memory and a large capacity HDD so as to utilize the nonvolatile solid-state memory as data storage area to be used only in a transitional state and the HDD as data storage area to be used in a steady state.